This invention resides generally in the fields of pyridine, silicon and catalyst chemistry. More particularly, it relates to the use of a siloxane having 4-dialkylamino functions incorporated into its polymer backbone as a catalyst for the hydrolysis of esters. This siloxane catalyst has shown remarkable selectivity and efficacy for the catalysis of esters, in some cases demonstrating enzyme-like substrate selectivity and catalytic efficacy.
By way of further background, 4-dialkylaminopyridines ("DAAP's") are highly nucleophilic and exhibit catalytic activity toward a variety of reactions including acylations of derivatives of carbon, phosphorous and sulfur acids, and by silylations, ester rearrangements, polymerizations, redox and other reactions. Early catalytic work with such DAAP compounds involved monomolecular species including, for instance, extensive work both commercially and in the literature with 4-dimethylaminopyridine (commonly referred to as "DMAP"). DMAP itself exhibits remarkable catalytic activity and has become a standard by which the activity of other DAAP compounds is often measured. For example, the efficacy of a subject DAAP compound in catalyzing a transacylation reaction of a sterically hindered alcohol such as 1-methylcyclohexanol with an anhydride such as acetic anhydride is commonly compared to that of DMAP. In such comparisons, the rate of the transacylation catalyzed by DMAP may be assigned a relative value of 1 (i.e., 100%), and the relative catalytic rate of the other DAAP compound is expressed as a fraction (i.e. percentage) thereof.
More recent work in this field has involved attempts to incorporate DAAP functionality into polymers while maintaining as much of its catalytic activity as possible. In so doing, the hope is that polymeric catalyst compositions may be formed which exhibit a wide range of valuable chemical, physical and dynamic-mechanical properties which prove more adaptable to varied uses than their monomeric counterparts.
For example, several vinyl-based polymeric catalysts with pendant DAAP groups have been prepared, including for instance: (1) poly[N-methyl-N-(4-vinylbenzyl)aminopyridine], Menger, F.M., McCann, D.J., J. Org. Chem. 1985, 50, 3928; (2) poly(diallylaminopyridine), Mathias, L.J., Vaidya, R.A., Bloodworth, R.H., J. Polym. Lett. Ed. 1985, 23, 147; Mathias, L.J., Vaidya, R.A., J. Am. Chem. Soc. 1986, 108, 1093; and Vaidya, R.A., Mathias, L.J., ibid 1986, 108, 5514; and (3) poly[methyl(3-styrenylpropyl)aminopyridine] crosslinked with divinylbenzene, Frechet, J.M.J., Darling, G.D., Itsuno, S., Lu, P., de Meftahi, M.N., Rolls Jr., W.A., Pure Appl. Chem. 1988, 60, 353. However, these vinyl-based catalyst materials have generally suffered in that they thermally degrade at temperatures below 300.degree. C. and thus cannot effectively be used at higher temperatures which are preferred for many reactions. Further, in many instances the polymer-supported DAAP functions have been appended to their vinyl backbones in ways that significantly decrease their catalytic activities.
In another aspect, certain polymeric DAAP materials have been reported to exhibit high nucleophilicity in aqueous solution, although reports to this point have been quite limited. For example, only two such reports are presently known to applicant, those involving derivatives of polyethylene imine, Delaney, E.J., Wood, L.E., Klotz, I.M., J. Am. Chem. Soc. 1982, 104,799, and poly[4-(pyrrolidino)pyridines], Vaidya, R.A., Mathias, L.J., J. Am. Chem. Soc. 1986, 108, 5514.
These and other considerations have represented significant drawbacks to this point in the development of truly satisfactory polymeric catalyst materials not only having effective DAAP functionality, but also exhibiting physical, chemical and dynamic-mechanical properties which make them applicable over a wide range of reactions and reaction conditions. Additionally, in one particular aspect, the need continues for further polymeric DAAP materials which exhibit desirable properties in aqueous mediums. The applicants' invention addresses these needs.